1. Field of the Invention
The present invention relates to a series of optical recording media made by preparing optical recording media of a plurality of types, the optical recording medium including a plurality of recording and reading layers.
2. Description of the Related Art
Conventionally, optical recording media such as CD-DAs, CD-ROMs, CD-Rs, CD-RWs, DVD-ROMs, DVD-Rs, DVD+/−RWs, DVD-RAMs, Blu-ray Discs (BD) are widely used to view digital video contents and to record digital data. In the BD standard, which is one of the next generation DVD standards, the wavelength of the laser beam used for recording and reading is reduced to 405 nm, and the numerical aperture of an objective lens is set to 0.85. In optical recording media conforming to the BD standard, tracks are formed at 0.32 μm pitch. This enables 25 GB or more data to be recorded on and read from one recording and reading layer of such an optical recording medium.
It is expected that the size of video and data files will increase more and more in the future. Therefore, it is contemplated to increase the capacities of optical recording media by using a multiple stack of recording and reading layers. In the technologies for BD standard optical recording media reported by I. Ichimura et al., Appl. Opt., 45, 1974-1803 (2006) and K. Mishima et al., Proc. of SPIE, 6282, 62820I (2006), 6 to 8 recording and reading layers are provided to achieve ultra large capacities as much as 200 GB.
When a multiple stack of recording and reading layers is used in an optical recording medium, projections and depressions, such as grooves and lands, for tracking control are formed on each recording and reading layer, and therefore a stamper (a master die) must be used to form the projections and depressions each time a recording and reading layer is formed. Therefore, as the number of stacked layers increases, the number of times the stamper is used increases, and this may result in an increase in manufacturing cost.
In the technologies for optical recording media recently proposed in Japanese Patent Application Laid-Open Nos. 2002-63738, and 2009-104717, servo layers are provided separately from recording and reading layers, and information is recorded on each recording and reading layer using a recording laser beam while a tracking signal is obtained from a corresponding one of the servo layers using a servo laser beam. With such technologies, the recording and reading layers need not have projections and depressions (grooves) for tracking information, and a stamper need not be used for the recording and reading layers during manufacturing. This enables a significant reduction in cost.
In the optical recording medium disclosed in Japanese Patent Application Laid-Open No. 2002-63738 or 2009-104717, the number of stacked recording and reading layers should be changed according to storage capacity required. However, if various optical recording media of different numbers of stacked recording and reading layers are produced, an optical pickup responsible for recording and reading of information cannot know the positions of recording and reading layers and of a servo layer in a direction of the thickness of each optical recording medium.
To be more specific, in the optical recording medium disclosed in Japanese Patent Application Laid-Open No. 2002-63738 or 2009-104717, change in the number of the stacked recording and reading layers changes the position of the servo layer depending on the thickness of a spacer layer and the like. Thus, each time an optical recording medium is set in place, the focus of a recording laser beam should be caused to move in a focus direction to know the positions of each recording and reading layer and of the servo layer. Accordingly, it takes a long time for preparation of recording and reading.
In this optical recording medium, an aberration generated by a difference in thickness of spacer layers between the recording and reading layers is corrected by a beam expander of an optical system of the optical pickup, and then recording and reading to and from each recording and reading layer are realized. Meanwhile, the optical pickup places limitations on the allowable amount of aberration correction by itself. Accordingly, if an optical recording medium has a larger number of layers in the future, recording and reading layers may be set outside the allowable range of aberration correction. Thus, an optical pickup may not be able to conduct aberration correction properly. Likewise, setting of a servo layer outside the allowable range of aberration correction by the optical pickup also makes the servo layer completely unable to conduct tracking control which is the most important process of recording and reading.